Salient-pole type linear motor and reciprocal double piston compressor with salient-pole type linear motor

ABSTRACT

A salient-pole type linear motor comprises a housing, a stator, a mover and coil units. The stator comprises a middle stator core and two side stator cores which have a plurality of magnetic poles on the inner peripheral surface of the stator. A plurality of coil units is wound around the plurality of magnetic poles. By generating a current that passes through the coil units sequentially, magnetic fields are produced about the magnetic poles and an attractive force is produced to enable a reciprocal motion of the mover by magnetic attraction. The salient-pole type linear motor further connects two compressor cylinders at both ends thereof for compressing gases or draining fluids so as to form a reciprocal double piston compressor with a salient-pole type linear motor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119 of TaiwanesePatent Application No. 101128744, filed Aug. 9, 2012, which is herebyincorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a salient-pole type linear motor and areciprocal double piston compressor with the salient-pole type linearmotor, in particular to the salient-pole type linear motor that usesmagnetic poles of a stator core arranged in front and behind a stator todrive coils on the front and rear magnetic poles to produced a magneticfield in order to attract a mover by the magnetic poles to perform areciprocal linear motion along a central axial direction of the stator.Two compressor cylinders are further connected to both ends of thesalient-pole type linear motor for compressing gases or draining fluidsto form a reciprocal double piston compressor with the salient-pole typelinear motor.

2. Description of the Related Art

In a conventional rotary switched reluctance motor (SRM), twomagnetically permeable materials in a magnetic circuit will take a pathwith the minimum magnetic reluctance when the lines of magnetic forcesare sealed, so that the two magnetically permeable materials in themagnetic circuit will move in a direction towards a position with theminimum magnetic reluctance to push the movable magnetically permeablematerials to the fixed magnetically permeable materials. To producemagnetic attractions with respect to one another, and the magneticattraction can serve as a driving torque source of the motor. Due to theprinciple of torque, the rotor and the stator of the motor are situatedin a salient pole status. No wiring with an exciting field or permanentmagnet is required on the rotor, and a simple structure of a core withgood magnetic permeability is used instead lower the cost of theassembly, and thus the magnetic reluctance motor has the advantages of asimple structure and a small volume that can be used in an applicationof high rotating speeds.

Conventional reciprocal compressors generally require a greater pushingforce of the motor to achieve the compression cycle of gases or drainingof the fluid. However, the conventional reciprocal compressors arelimited by the total volume of the compressor, and the generalcylindrical linear motor cannot meet the requirement for the highdriving forces. Only the conventional reciprocal compressors can meetthe requirements of such a high driving force, and the compressors ofthis kind mainly use a rotary motor to drive a crank, and a link rodmechanism to push a piston, so that the gas in the cylinder can becompressed or the fluid in the cylinder can be drained, so as to obtainthe required pressure.

Although the reciprocal compressor driven by the conventional rotarymotor has the advantages of low cost and a broad operation range, yet alateral force may be produced during the compression process of thepiston due to the linear motion of the piston driven by the rotarymotor, and thus the wall of the cylinder may be worn out or damagedeasily to indirectly causing the problems of noise, vibrations and highmechanical losses, so as to affect the overall system performanceadversely. Although the problem can be resolved by applying a lubricant,the lubricant may contaminate the compressed gas or fluid.

BRIEF SUMMARY

In view of the shortcomings of the prior art, it is a primary objectiveof the present invention to provide a salient-pole type linear motor anda reciprocal double piston compressor with the salient-pole type linearmotor, wherein magnetic poles of a stator are arranged at the front andrear of the stator and a coil installed on the magnetic pole is alsoprovided for driving the salient-pole type mover to perform a linearreciprocal motion, so as to drive a piston installed separately at bothends of a compressor. The linear motor of the present invention isprovided for driving the compressor, not just creating a high magneticflux density of the coil on the magnetic poles of the stator and astrong magnetic driving force only, but also featuring a simplestructure of the salient-pole mover made of a material with goodmagnetic permeability or a permanent magnet, so as to simplify thequantity of components and reduce manufacturing costs. In addition, themotion direction of the mover is a linear reciprocal motion, which canreduce the lateral force produced from the motion of the piston andfriction and loss to achieve the effects of high mechanical strength,low mechanical loss, low noise levels and high efficiency.

To achieve the foregoing objective, the present invention provides asalient-pole type linear motor comprising a housing, a stator, a moverand a plurality of coil units.

Wherein, the stator is installed in the housing. The stator comprises amiddle stator core and two side stator cores coupled along a centralaxial direction. Wherein, the middle stator core and the two side statorcores further have a plurality of magnetic poles paired and protrudedinwardly. In addition, the middle stator core has the magnetic polesdisposed at positions corresponding to the positions of the magneticpoles of the two side stator cores respectively, so that the magneticpoles of the two side stator cores are staggered or arranged opposite toeach other along the central axial direction, so as to wind theplurality of coil units on the front and rear magnetic poles. Inaddition, the middle stator core is selectively and integrally formedwith the two side stator cores or the middle stator core is integrallyformed with one of the two side stator cores.

Further, the mover of the salient-pole type linear motor has a shaft anda plurality of salient-poles. Wherein, the shaft is movably penetratedalong the central axial direction through the stator. The number ofsalient-poles is equal to the number of magnetic poles of the middlestator core, and the position of the salient-pole is corresponsive tothe position of the magnetic pole of the middle stator core. When acurrent is applied sequentially at the coil units, a magnetic field isproduced by the coil units wound around the magnetic poles to drive themagnetic poles, in order to attract the mover to perform a reciprocallinear motion along the central axial direction of the stator.

In the salient-pole type linear motor, the length of the stator can beextended in a unit of the middle stator core and/or two side statorcores, and the length of the salient pole of the mover can be increasedto increase the motion stroke of the mover, and the number of salientpoles of the mover and the number of salient poles of the stator can beincreased to strengthen the magnetic attraction of the magnetic pole tothe mover, so as to enhance the driving force of the motor effectively.

A further objective of the present invention is to provide a reciprocaldouble piston compressor with a salient-pole type linear motor. Thereciprocal double piston compressor with a salient-pole type linearmotor comprises a housing, a stator, a mover, a plurality of coil unitsand a plurality of compressor cylinders.

The salient-pole type linear motor can be coupled to a plurality ofcompressor cylinders with a piston. Wherein, the piston of thecompressor cylinder is coupled separately to both ends of the shaft ofthe mover, so that the gas in the compressor cylinder can be compressed,or the fluid in the compressor cylinder can be drained. Electric energypassed into the driving coils at the front and rear magnetic poles ofthe stator core of the salient-pole type linear motor is converted intomechanical energy of the linear motion through magnetic energy, withoutrequiring any additional conversion and transmission mechanism orcomponent. The mover is driven directly for the reciprocal linearmotion, thus not only reducing the lateral force produced from thepiston motion, but also reducing friction and loss, so as to furtherachieve the effects of high mechanical strength, low mechanical loss,low noise and high efficiency.

In summation, the reciprocal double piston compressor of thesalient-pole type linear motor of the present invention has one or moreof the following advantages:

(1) In the salient-pole type linear motor of the present invention, thelength of the stator can be extended in the unit of a middle stator coreand two side stator cores, and the length of the salient pole of themover can be increased in order to increase the motion stroke of themover.

(2) In the salient-pole type linear motor of the present invention, thenumber of salient poles of the mover and the number of magnetic poles ofthe stator core can be increased to strengthen the magnetic attractionof the magnetic pole to the mover, so as to enhance the driving force ofthe motor effectively.

(3) In the salient-pole type linear motor of the present invention, aplurality of compressor cylinders with a piston can be coupled, suchthat the reciprocal linear motion of the mover of the salient-pole typelinear motor can be used to reduce the lateral force produced during thepiston motion and also reduce friction and loss, so as to furtherachieve the effects of high mechanical strength, low mechanical loss,low noise and high efficiency.

(4) In the salient-pole type linear motor of the present invention, ahigh magnetic flux density of the coil is formed on the stator magneticpole, and a strong magnetic driving force is produced. In addition, thesalient pole mover is made of a material with good magnetic permeabilityor a permanent magnet to simplify the components and reducesmanufacturing costs significantly.

(5) With the design of the salient-pole type linear motor separated fromthe two side compressors, the compressed gas or liquid of the two sidecompressors does not flow through the motor, so that the magnetic forceof the motor will not be affected by the high-temperature of thecompressed gas or liquid that leads to a loss of driving power.Therefore, the linear motor can be designed to meet differentrequirements of dimensions and compression forces and can be used forthe transmission of special hazardous chemical gases or fluids.

(6) The salient-pole type linear motor of the present invention comeswith simple components, and the stator coil provided for driving themotion of the salient pole mover has a very high linearity of the directlinear reciprocal motion, so that the compressor can be designed aslubricant-free, which is a design with excellent energy-saving effectand high efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a salient-pole type linear motor of thepresent invention;

FIG. 2 is a front view of a salient-pole type linear motor of thepresent invention;

FIG. 3A is a schematic view of a salient-pole type linear motor with acurrent applied to a first coil unit of the present invention;

FIG. 3B is a schematic view of a salient-pole type linear motor with acurrent applied to a second coil unit of the present invention;

FIG. 4A is a schematic view of a salient-pole type linear motor having astator with an extended length and a current applied to a third coilunit of the present invention;

FIG. 4B is a schematic view of a salient-pole type linear motor having astator with an extended length and a current applied to a first coilunit of the present invention;

FIG. 4C is a schematic view of a salient-pole type linear motor having astator with an extended length and a current applied to a second coilunit of the present invention;

FIG. 5 is a cross-sectional view of a salient-pole type linear motorwith both ends coupled to a compressor cylinder of the presentinvention;

FIG. 6A is a cross-sectional view of a salient-pole type linear motorhaving two sets of double piston compressors serially connected to apiston of the present invention, showing its motion towards both sides;

FIG. 6B is a cross-sectional view of a salient-pole type linear motorhaving two sets of double piston compressors serially connected to apiston of the present invention, showing its motion towards the middle;and

FIG. 7 is a schematic view of a salient-pole type linear motor withenhanced magnetic forces at the ends.

DETAILED DESCRIPTION

The technical characteristics of the present invention will becomeapparent with the detailed description of the preferred embodimentsaccompanied with the illustration of related drawings as follows. It isnoteworthy that same numerals are used for representing the samerespective elements in the drawings, and the drawings are provided forthe purpose of illustrating the invention, but not intended for limitingthe scope of the invention.

In a salient-pole type linear motor of the present invention, magneticpoles of front and rear stator cores are disposed on a stator, and acoil is formed on the magnetic pole, such that a magnetic field producedby the front and rear coils will go through a path with a minimummagnetic reluctance by sealing the magnetic lines of force, such thatthe mover will be moved in a direction towards the minimum magneticreluctance, so as to constitute the salient-pole type linear motor of alinear motion. In addition, the salient-pole type linear motor adopts acompressor with the design of driving a fluid or compressing a gas bydouble pistons through double paths to replace the conventional rotarymotor operated together with a link rod mechanism for driving thepiston.

With reference to FIG. 1 for an exploded view of a salient-pole typelinear motor of the present invention, the salient-pole type linearmotor comprises a housing 400, a stator, a mover 300 and a plurality ofcoil units.

Wherein, the stator is installed in the housing 400 and further includesa plurality of heat dissipating elements 410 installed thereon, and theheat dissipating elements 410 can be fins or any other componentscapable of dissipating the heat from the housing 400. In addition, aplurality of screws 430 and a cover 420 are provided for covering bothends of the housing 400. The stator is comprised of a middle stator core100, a front stator core 110 and a rear stator core 120 and coupledalong the central axial direction of the stator. The middle stator core100, the front stator core 110 and the rear stator core 120 are made ofiron, silicon steel or any other magnetically permeable material.Further, the middle stator core 100 can be selectively and integrallyformed with the front stator core 110 and the rear stator core 120, orthe middle stator core 100 is integrally formed with the front statorcore 110 or the rear stator core 120. In other words, the salient-poletype linear motor of the present invention may not include the middlestator core 100, and magnetic poles of the front stator core 110 and therear stator core 120 can be staggered or arranged in opposite positionswith each other along the central axial direction.

The middle stator core 100 further includes a first magnetic pole unit101 and a second magnetic pole unit 102 paired and protruded inwardly,and the quantities of the first magnetic pole units 101 and the secondmagnetic pole units 102 can be two, three, or four respectively, but thepresent invention is not limited to such quantities only. The frontstator core 110 and the rear stator core 120 have a third magnetic poleunit 111 and a fourth magnetic pole unit 121 paired and protrudedinwardly and respectively, wherein the quantities of the third magneticpole unit 111 and the fourth magnetic pole unit 121 can be two, three,or four respectively, but the present invention is not limited to suchquantities only. In addition, the quantity of magnetic poles of themiddle stator core 100 can be equal to the total quantity of magneticpoles of the front stator core 110 and the rear stator core 120. Inother words, the total quantity of the first magnetic pole units 101 andthe second magnetic pole units 102 is equal to the total quantity of thethird magnetic pole units 111 and the fourth magnetic pole units 121.

Further, the position of the first magnetic pole unit 101 iscorresponsive to the position of the fourth magnetic pole unit 121, andthe position of the second magnetic pole unit 102 is corresponsive tothe position of the third magnetic pole unit 111. Therefore, the thirdmagnetic pole unit 111 and the fourth magnetic pole unit 121 can bestaggered along the central axial direction of the stator. In addition,the first coil unit 130 is wound around the second magnetic pole unit102 and the third magnetic pole unit 111, and the second coil unit 140is wound around the first magnetic pole unit 101 and the fourth magneticpole unit 121.

Further, the mover 300 of the salient-pole type linear motor has a shaft320 and a plurality of salient poles 310. Wherein, the mover 300 is madeof iron, silicon steel, a permanent magnet or any other magneticallypermeable material. The number of salient poles 310 is equal to thenumber of magnetic poles of the middle stator core 100. In other words,if the total quantity of the first magnetic pole units 101 and thesecond magnetic pole units 102 is equal to four, then the quantity ofsalient poles 310 is also equal to four. Wherein, the shaft 320 ismovably penetrated through the stator along the central axial direction,and the shaft 320 is protruded from the cover 420. The position of thesalient pole 310 is corresponsive to the positions of the first magneticpole unit 101 and the second magnetic pole unit 102.

Therefore, when a current is applied to the first coil unit 130, themagnetic field produced by the first coil unit 130 wound around thesecond magnetic pole unit 102 and the third magnetic pole unit 111 canattract the mover 300 to move along the central axial direction of thestator towards the positions of the second magnetic pole unit 102 andthe third magnetic pole unit 111. On the other hand, when a current isapplied to the second coil unit 140, the magnetic field produced by thesecond coil unit 140 wound around the first magnetic pole unit 101 andthe fourth magnetic pole unit 121 can attract the mover 300 to movealong the central axial direction of the stator towards the positions ofthe first magnetic pole unit 101 and the fourth magnetic pole unit 121.Therefore, when the current is applied sequentially to the first coilunit 130 and the second coil unit 140, the magnetic pole is driven toattract the mover 300 to perform a reciprocal linear motion in thecentral axial direction of the stator.

In the salient-pole type linear motor, the length of the stator can beextended in a unit of the middle stator core 100, the front stator core110 and/or the rear stator core 120, and the length of the salient pole310 of the mover 300 is also increased to increase the motion stroke ofthe mover 300. In addition, the salient-pole type linear motor can havean increased number of salient poles 310 of the mover 300 and anincreased number of magnetic poles of the stator core, so as to enhancethe magnetic attraction of the magnetic pole to the mover 300 andimprove the driving force of the motor effectively.

With reference to FIGS. 1 and 2, FIG. 2 shows the front view of thesalient-pole type linear motor of the present invention. For simplicity,the housing 400, the heat dissipating element 410, the cover 420 and thescrew 430 are not shown in the figure.

Further, the first coil unit 130 of the salient-pole type linear motoris wound around the second magnetic pole unit 102 and the third magneticpole unit 111, and the second coil unit 140 is wound around the firstmagnetic pole unit 101 and the fourth magnetic pole unit 121. If currentis applied sequentially to the first coil unit 130 and the second coilunit 140, the magnetic force will drive the magnetic pole to attract themover 300 to perform a reciprocal linear motion along the central axialdirection of the stator.

With reference to FIGS. 3A and 3B for schematic views of a salient-poletype linear motor of the present invention respectively, after thesalient-pole type linear motor is assembled, the second magnetic poleunit 102 and the third magnetic pole unit 111 jointly constitute thefirst magnetic pole 210, and the first magnetic pole unit 101 and thefourth magnetic pole unit 121 jointly constitute the second magneticpole 220.

In FIG. 3A, when a current is applied to the first coil unit 130, themagnetic field produced by the first coil unit 130 wound around thefirst magnetic pole 210 attracts the mover 300 to move along the centralaxial direction of the stator towards the position of the first magneticpole 210. In FIG. 3B, when the current is applied to the second coilunit 140, the magnetic field produced by the second coil unit 140 woundaround the second magnetic pole 220 attracts the mover 300 to move alongthe central axial direction of the stator towards the position of thesecond magnetic pole 220. When the current is applied sequentially tothe first coil unit 130 and the second coil unit 140, a magnetic flux ispassed through the magnetic pole to attract the mover 300 to perform areciprocal linear motion along the central axial direction of thestator.

With reference to FIGS. 3A, 3B, and 4A to 4C, FIGS. 4A to 4C areschematic views of a salient-pole type linear motor extended with thelength of a stator in accordance with the present invention. Forsimplicity, the housing 400, the heat dissipating element 410, the cover420 and the screw 430 are not shown in the figure.

Further, before the third magnetic pole 200 of the salient-pole typelinear motor is coupled to the first magnetic pole 210, the third coilunit 150 is wound around the third magnetic pole 200. In FIG. 4A, when acurrent is applied to the third coil unit 150, the magnetic fieldproduced by the third coil unit 150 wound around the third magnetic pole200 attracts the mover 300 to move along a central axial directiontowards the position of the third magnetic pole 200 of the stator.

In FIG. 4C, when a current is applied to the first coil unit 130, themagnetic field produced by the first coil unit 130 wound around thefirst magnetic pole 210 attracts the mover 300 to move along the centralaxial direction of the stator towards the position of the first magneticpole 210.

In FIG. 4C, when a current is applied to the second coil unit 140, themagnetic field produced by the second coil unit 140 wound around thesecond magnetic pole 220 attracts the mover 300 to move along thecentral axial direction of the stator towards the position of the secondmagnetic pole 220.

Therefore, when the current is applied sequentially to the first coilunit 130, the second coil unit 140 and the third coil unit 150, themagnetic pole is driven to attract the mover 300 to perform a reciprocallinear motion along the central axial direction of the stator, so as toenhance the motion stroke of the mover 300.

With reference to FIG. 5 for a cross-sectional view of a salient-poletype linear motor with both ends coupled to a compressor cylinder of thepresent invention, a mover 300 with eight salient poles 310 is used asan example for illustrating the present invention, the salient-pole typelinear motor is further coupled to a compressor cylinder 500 with apiston 510, and the compressor cylinder 500 further includes a pluralityof heat dissipating elements 410 which can be fins or any othercomponent capable of dissipating heat from the housing 400. The piston510 of the compressor cylinder 500 is coupled separately to both ends ofthe shaft 320 of the mover 300 and provided for compressing a gas ordraining a fluid in the compressor cylinder 500.

The compressor cylinder 500 further comprises four intake check valves520 and four outlet check valves 530. When a current is applied to thefirst coil unit 130, the first magnetic pole 210 attracts the mover 300to drive the piston 510 on both sides to compress towards the left side,so as to open the outlet check valve 530 disposed at the lower left ofthe compressor cylinder 500, and compress the gas or the fluid anddischarge from the outlet check valve 530 disposed at the lower left ofthe compressor cylinder 500. In the mean time, the intake check valve520 disposed on the upper right of the two side compressor cylinders 500will be opened due to the negative pressure formed inside the compressorcylinder 500, so that the gas or the fluid can enter from the intakecheck valve 520 at the upper right of the compressor cylinder 500 intothe compressor cylinder 500.

When a current is applied to the second coil unit 140, the secondmagnetic pole 220 attracts the mover 300 to drive the piston 510 on bothsides to compress at the right side, so as to open the outlet checkvalve 530 disposed at the lower right of the two side compressorcylinders 500 and compress the gas or fluid to discharge out from theoutlet check valve 530 at the lower right of the compressor cylinder500. In the meantime, the intake check valve 520 at the upper left ofthe two side compressor cylinders 500 is opened due to the negativepressure formed inside the compressor cylinder 500, so that the gas orfluid can enter from the intake check valve 520 at the upper left of thecompressor cylinder 500 into the compressor cylinder 500.

Therefore, the reciprocal linear motion of the mover 300 of thesalient-pole type linear motor not only can reduce the lateral forceproduced from the motion of the piston 510, but can also reduce frictionand loss, so as to achieve the effects of high mechanical strength, lowmechanical loss, low noise and a high efficiency. The linear motor ofthe present invention drives the reciprocal double piston compressor canbe designed with a high-low compressor model, and applied to provide adriving source for air conditioners of electric cars, dehumidifiers,cold drink machines, water chillers, refrigerators, homeair-conditioners, refrigeration compressors, electric appliances,air-conditioning compressor, air-conditioner compressors, industrialcompressors or any system requiring a reciprocal motion compression.

With reference to FIGS. 6A and 6B, both ends of two salient-pole typelinear motors 500 can be connected in series with the compressorcylinder in order to inhibit vibrations produced by the salient-poletype linear motor with both ends coupled to the compressor cylinder 500,and two sets of pistons 510 are arranged to have motions with respect toeach other to offset the vibrations caused by the reciprocal motion ofthe pistons, so as to achieve the effect of balancing the force toreduce vibrations.

With reference to FIG. 7 for a perspective view of a salient-pole typelinear motor with enhanced magnetic forces at the ends, a mover 300 witheight salient poles 310 is used as an example for the illustration ofthe invention. A fourth magnetic pole 230 is further installed at afront end of the second magnetic pole 220, and the fourth coil unit 160is wound around the fourth magnetic pole 230. In addition, a fifthmagnetic pole 240 is installed at an end of the first magnetic pole 210and the fifth coil unit 170 is wound around the fifth magnetic pole 240.

Further, when a current is applied to the first coil unit 130 and thefourth coil unit 160, the first magnetic pole 210 and the fourthmagnetic pole 230 attract the mover 300 to move along the central axialdirection of the stator towards the positions of the first magnetic pole210 and the fourth magnetic pole 230. On the other hand, when thecurrent is applied to the second coil unit 140 and the fifth coil unit170, the second magnetic pole 220 and the fifth magnetic pole 240attract the mover 300 to move along the central axial direction of thestator towards the positions of the second magnetic pole 220 and thefifth magnetic pole 240.

Therefore, the fourth magnetic pole 230 and the fifth magnetic pole 240are provided and disposed at the front and rear ends of the salient-poletype linear motor respectively to enhance the magnetic forces at thefront and rear ends of the magnetic force, so as to improve the drivingforce at both ends of the motor.

What is claimed is:
 1. A salient-pole type linear motor, comprising: a housing; a stator, installed in the housing, and comprising a middle stator core and two side stator cores coupled along a central axial direction, and the middle stator core and the two side stator cores further having a plurality of magnetic poles paired and protruded inwardly; a plurality of coil units, wound around the magnetic poles; and a mover, having a shaft and a plurality of salient-poles, and the shaft movably penetrating along the central axial direction through the stator, such that when current is applied sequentially at the coil units, a magnetic field is produced by the coil units wound around the magnetic poles to drive the magnetic poles to attract the mover to perform a reciprocal linear motion along the central axial direction of the stator.
 2. The salient-pole type linear motor of claim 1, wherein the middle stator core has the magnetic poles disposed at positions corresponding to the positions of the magnetic poles of the two side stator cores respectively, so that the magnetic poles of the two side stator cores are staggered or arranged opposite to each other along the central axial direction.
 3. The salient-pole type linear motor of claim 1, wherein the mover has a number of salient poles equal to the number of magnetic poles of the middle stator core, and the salient poles are disposed at positions corresponding to the positions of the magnetic poles of the middle stator core respectively, and the middle stator core and the two side stator cores are made of iron, silicon steel or a magnetically permeable material.
 4. The salient-pole type linear motor of claim 1, wherein the mover is made of iron, silicon steel, a permanent magnet or a magnetically permeable material.
 5. The salient-pole type linear motor of claim 1, wherein the middle stator core and the two side stator cores are integrally formed, or the middle stator core is integrally formed with one of the two side stator cores, and the magnetic poles of the two side stator cores are staggered or arranged opposite to each other along the central axial direction.
 6. A reciprocal double piston compressor with a salient-pole type linear motor, comprising: a housing; a stator, installed in the housing, and comprising a middle stator core and two side stator cores coupled along a central axial direction of the stator, and the middle stator core and the two side stator cores having a plurality of magnetic poles paired and protruded inwardly; a plurality of coil units, wound around the magnetic poles; a mover, having a shaft and a plurality of salient-poles, and the shaft movably penetrating through the stator along the central axial direction, such that when current is applied sequentially at the coil units, a magnetic field is produced by the coil units wound around the magnetic poles to drive the magnetic poles to attract the mover to perform a reciprocal linear motion along the central axial direction of the stator; and a plurality of compressor cylinders, each having a piston coupled separately to both ends of the shaft of the mover.
 7. The reciprocal double piston compressor with a salient-pole type linear motor according to claim 6, wherein the middle stator core has the magnetic poles disposed at positions corresponding to the positions of the magnetic poles of the two side stator cores respectively, so that the magnetic poles of the two side stator cores are staggered or arranged opposite to each other along the central axial direction.
 8. The reciprocal double piston compressor with a salient-pole type linear motor according to claim 6, wherein the mover has a number of salient poles equal to the number of magnetic poles of the middle stator core, and the salient poles are disposed at positions corresponding to the positions of the magnetic poles of the middle stator core respectively, and the middle stator core and the two side stator cores are made of iron, silicon steel or a magnetically permeable material.
 9. The reciprocal double piston compressor with a salient-pole type linear motor according to claim 6, wherein the compressor cylinders further comprise a plurality of intake check valves and a plurality of outlet check valves.
 10. The reciprocal double piston compressor with a salient-pole type linear motor according to claim 6, wherein the middle stator core and the two side stator cores are integrally formed, or the middle stator core is integrally formed with one of the two side stator cores, and the magnetic poles of the two side stator cores are staggered or arranged opposite to each other along the central axial direction. 